title: Competition Between Extinction and Enhancement in Surface Enhanced Raman Spectroscopy.
abstract: Conjugated metallic nanoparticles are a promising means to achieve ultrasensitive and multiplexed sensing in intact three-dimensional samples, especially for biological applications, via surface enhanced Raman scattering (SERS). We show that enhancement and extinction are linked and compete in a collection of metallic nanoparticles. Counterintuitively, the Raman signal vanishes when nanoparticles are excited at their plasmon resonance, while increasing nanoparticle concentrations at off-resonance excitation sometimes leads to decreased signal. We develop an effective medium theory that explains both phenomena. Optimal choices of excitation wavelength, individual particle enhancement factor and concentrations are indicated. The same processes which give rise to enhancement also lead to increased extinction of both the illumination and the Raman scattered light. Nanoparticles attenuate the incident field (blue) and at the same time provide local enhancement for SERS. Likewise the radiation of the Raman-scattered field (green) is enhanced by the near-by sphere but extinguished by the rest of the spheres in the suspension on propagation.
authors.affiliation: Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA, Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA, Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA, Department of Bioengineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA, and Department of Mechanical Science and Engineering, Chemical and Biomolecular Engineering and University of Illinois Cancer Center, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA.
authors.forename: Thomas
authors.initials: T
authors.lastname: van Dijk
authors.forename: Sean T
authors.initials: ST
authors.lastname: Sivapalan
authors.forename: Brent M
authors.initials: BM
authors.lastname: Devetter
authors.forename: Timothy K
authors.initials: TK
authors.lastname: Yang
authors.forename: Matthew V
authors.initials: MV
authors.lastname: Schulmerich
authors.forename: Catherine J
authors.initials: CJ
authors.lastname: Murphy
authors.forename: Rohit
authors.initials: R
authors.lastname: Bhargava
authors.forename: P Scott
authors.initials: PS
authors.lastname: Carney
keywords: Beer’s law; SERS; deep-tissue imaging; medical imaging; nanoparticles; spectroscopy; suspension